Neutral Oxygen Vacancy Defects Research Articles

Demonstration of electronic and optical synaptic properties modulation of reactively sputtered zinc-oxide-based artificial synapses

A stable and highly controllable multistate analog memory system was developed using ZnO-based memristors. Indium–tin oxide (ITO)/ZnO/ITO memristors exhibited electrical and optical synaptic properties similar to those of biological synapses. Multilevel conductivity was achieved by controlling the current compliance and RESET stop voltage for high-density memory storage and learning applications. Results of high-resolution transmission electron microscopy indicated that polycrystalline ZnO was formed during the reactive sputtering of Zn with O2. The memristor could realize multilevel memory storage owing to the continuous formation of neutral oxygen vacancy defects. Biological synaptic rules were mimicked and implemented by modifying and analyzing the excitatory post-synaptic current of the memristor. Furthermore, short- and long-term memory applications during paired-pulse facilitation, spike-number-dependent plasticity, and pulse rate-dependent plasticity were demonstrated. The light-controlled synaptic weight was modulated through the charge carrier generation inside the ZnO switching layer below the valence band. Repeated rehearsals of the optical pulse resulted in short-term plasticity and long-term potentiation. The transition occurred depending on the pulse intensity and number, resulting from the dynamic generation and trapping of photogenerated carriers. Short- and long-term memory functions for the learning and forgetting processes of artificial visual memory systems were accomplished by varying the optical illumination intensity. The proposed ZnO-based memristors can be applied to electronic and photonic devices.

Density functional theory study of oxygen vacancy defect diffusion properties in λ-Ta2O5

The formation energy and diffusion properties of oxygen vacancy defects in λ-Ta2O5 were studied using first-principle calculations. The calculated formation energy of neutral oxygen vacancy defects was consistent with previous studies carried out, and the charged oxygen defects considering periodic correction were also studied. The calculations obtained showed that the formation energy of +2 charged oxygen vacancy defects was 0.83–1.16 eV. Also, the diffusion activation energy consisting of oxygen vacancy defect formation energy and diffusion barrier concurred well with the diffusion activation energy measured by experiments. A linear relationship was found between the diffusion barrier and the diffusion distance.

Quasi-particle calculations on electronic and optical properties of the peroxy linkage and neutral oxygen vacancy defects in amorphous silica

Recently, fused silica has been used to prepare the optical windows in the inertial confinement fusion (ICF) equipment. Challenge of application of fused silica is due to the defect-related optical absorption which is considered as the main mechanism of laser-induced damage process. However, due to structural complexity, calculation of the defect-related absorption from the first principles is only limited to small clusters, and a full treatment using the state of art GW and Bathe-Salpeter equation (BSE) method is still lacking.In this work, density functional theory calculations are performed to study the defect structure of the peroxy linkage (POL) and the neutral oxygen vacancy (NOV) defects in amorphous silica. Firstly, well relaxed structure is generated by using a combination of the bond switching Monte Carlo technique and the DFT-based structure optimization. Secondly, the defect structures are generated and studied in both the ground singlet (S0) and the first excited triplet (T1) states. Finally, the electronic and optical properties of the considered structures are studied by applying the self-consistent quasi-particle GW (sc-QPGW) and the BSE methods in Tamm-Dankoff approximation.In the ground state S0, the POL defect is found to be stable and shares a similar local structure to the H2O2 molecule. However, in T1 state, the POL defect breaks into a pair of E' center ( - Si ) and peroxy oxygen radial ( O-O-Si-). For the NOV defect, the optimized Si-Si bond length in the ground state is 2.51 with a variation of 0.1 due to the structural disorder. In comparison to the ground state, the optimized Si-Si bond length in T1 state increases to 3.56 .The scGW/BSE calculation on the defect free structure predicts a quasi particle band gap of 10.1 eV and an optical band gap of 8.0 eV, which are consistent well with the available experimental results. For the POL defect, the scGW/BSE calculation reveals a weak exciton peak at 6.3 eV. Below 6.3 eV, no new exciton peak is found, implying that the experimentally suggested 3.8 eV peak could not be attributed to the POL defect. Calculations of the NOV defect gives a strong and highly polarized optical absorption peak at 7.4 eV which is close to the previous experimental result at 7.6 eV. The structural relaxation induced by NOV also contributes to another absorption peak at 7.8 eV.

Effects of the Bombardment of Negative Oxygen Ions on Gate Bias Stability of InGaZnO Thin Film Transistors

During the sputtering process, the bombardment effect of negative oxygen ions (NOIs) on the bias stability of amorphous indium gallium zinc oxide based thin film transistors (a-IGZO TFTs) was investigated. To control the bombardment energy of NOIs, a-IGZO active layer was prepared by using a superimposed rf/dc magnetron sputter. The electrical characteristics of a-IGZO TFTs under a negative bias illumination stress (NBIS) shows the different trend with that in the dark. We propose that the heavy defects such as neutral oxygen vacancy defects (Vo) as bulk defects are closely related with the bombardment energy of the negative oxygen ions.

Defects in 700 keV oxygen ion irradiated ZnO

It is well known that energetic oxygen ions induce heavy crystalline disorder in ZnO, however, systematic study on this regard is very much limited. Here, we present photoluminescence (PL), optical absorption and sheet resistance measurements on poly and single crystalline ZnO samples irradiated with 700keV O ions. Results have been compared with the effects of 1.2MeV Ar irradiation on similar ZnO target. Colour change of the samples with increasing O irradiation fluence has also been noted. Non-monotonic variation of room temperature sheet resistance with the increase of fluence has been observed for polycrystalline ZnO. Such an outcome has been understood as point defects transforming to bigger size clusters. Near band edge (NBE) PL emission is largely reduced due to O ion irradiation. However, at 10K NBE emission can be observed for irradiated polycrystalline samples. Irradiated ZnO single crystal does not show any band to band transition even at 10K. It is evident that dynamic recovery of defects is more effective in polycrystalline samples. Ultraviolet–visible absorption spectrum of the irradiated ZnO crystal show pronounced sub-band gap absorption. Oxygen irradiation generated new absorption band in ZnO is at 3.05eV. In the light of earlier reports, this particular band can be ascribed to absorption by neutral oxygen vacancy defects.

White Electroluminescence from a Gadolinium-Doped Si-Nanocluster-Enriched SiO[sub 2]–SiON Interface Region

SiO2 implanted with carbon and silicon. In this article, we present bright white light emitters based on the silicon-rich SiO2‐SiON interface in a metal‐SiON‐SiO2‐Si MONOS structure coimplanted with gadolinium Gd. At a certain level of the constant injection current excitation, the MONOS structure containing Si-nc and Gd works as a white light emitter. At a constant current excitation of 100 A, the EL spectrum covers a wide spectral region from 360 to 700 nm with a broad flat region of about 200 nm. The increase in the constant current excitation switches the color of the light emitters observed by the naked eye from green to blue. For constant current excitation above 1 mA, the energy transfer from the Gd 3+ ions to the neutral oxygen vacancy defects and the silicon nanoclusters was observed. The photoluminescence PL measurements taken from only silicon implanted structures show two broad-bands with peaks at 640 and 780 nm. In Gd coimplanted Si-rich MONOS structures, the PL spectrum consists of one peak at 650 nm related to a nonbridging oxygen hole center NBOHC and two peaks at 760 and 830 nm associated with silicon nanoclusters with sizes of 1.7 and 1.8 nm, respectively. EL devices, metal‐SiON‐SiO2‐Si light emitting device MONOSLED, were prepared by a standard silicon MOS technology on 4 inch, n-type silicon 100 wafers with resistivities of 2‐5 cm. Thermally grown 100 nm thick SiO2 layers with a 1000 nm field oxide were processed using the local oxidation of silicon technology and were implanted by silicon and coimplanted with Gd at energies of 20 and 140 keV with atomic concentrations of 10 and 2%, respectively. Samples were annealed at 1100°C for 30 min after silicon implantation to form Si-nc and at 900°C for the same time after Gd coimplantation to reduce the radiation defects and to rebuild Si-nc. To protect the oxide layer against breakdown, a 200 nm SiON layer was deposited on top of it O:N =1 :1 by plasmaenhanced chemical vapor deposition. The atomic composition of the SiON layers and the concentration of implanted elements were confirmed by scanning Auger electron spectroscopy Microlab 310F. The gate and bottom electrodes consist of a 100 nm thick indium tin oxide ITO layer deposited by radio frequency sputtering and a 300 nm thick evaporated Al layer, respectively. Cross-sectional transmission electron microscopy images were taken by means of an FEI Titan 80-300 scanning transmission electron microscope operating at 300 keV. The EL measurements were performed using electron injection mainly from the ITO electrode. The EL spectra and the quenching of the EL intensity were measured at room temperature on MONOS structures with a circular ITO electrode of 300 m diameter at a constant current supplied by a source meter Keithley 2410. The EL signal was recorded with a monochromator Jobin Yvon Triax 320 and a photomultiplier Hamamatsu H7732-10 .P L experiments were conducted at room temperature with an excitation wavelength of 514.5 nm from an argon laser with a power density of 0.2 mW/mm 2 . The PL signal was recorded with an SPM-2 monochromator and a photomultiplier Hamamatsu R943-02. The photograph was taken by a standard digital camera combined with a microscope. Figure 1 shows the PL spectra taken from Si-nc’s embedded in the SiO2 layer and from the SiO2 layer containing Si-nc’s coimplanted with Gd. In the silicon implanted oxide layer with subsequent annealing at 1100°C, the PL signal shows a broad spectrum with two main bands centered at around 650 and 800 nm. The Gaussian peak deconvolution inset of Fig. 1 enables us to show

Photoluminescence and self-interference in germanium-doped silica films

Germanium-doped silica films were prepared by magnetron cosputtering and postannealing. The photoluminescence properties and their dependence on the Ge contents and annealing temperature were investigated. Our experiments indicate that the observed light emission originates from the neutral oxygen vacancy defects. The substructures in the luminescence bands of the films were found to result from multiple-beam interferences of the emission in the optical cavity formed by the transparent films.

Optical characterization of CO2-laser-ablated Si-rich SiOx

Anomalous absorption and the corresponding change in the optical band gap of a CO2-laser-ablated Si-rich SiO2 (SiOx) film are studied. The optical band gap energy of as-grown nonstoichiometric SiOx is slightly reduced by increasing Si–Si bonds as compared to quartz. After rapid thermal annealing using a CO2 laser, the dehydrogenation of SiOx film further increases the Si–Si bonding states and redshifts the optical band gap by 1eV. Laser ablation is initiated at a laser intensity of >7.5kW∕cm2, leaving numerous luminescent centers that are related to neutral oxygen vacancy defects, increasing the absorption coefficient and related optical band gap energy, and reducing the refractive index in partially annealed SiOx.

Time-Resolved Photoluminescence Analysis of Multidose Si-Ion-Implanted SiO2

The continuous-wave/time-resolved photoluminescence (CWPL/TRPL) and capacitance–voltage (C-V) analysis of multirecipe silicon-ion-implanted are demonstrated to study the lifetime evolution of three radiative defects with luminescent peaks at 415, 455, and , which are identified as the weak oxygen bonding (O–O) defects, neutral oxygen vacancy (NOV), and precursors of nanocrystallite Si defects, respectively. The TRPL analysis reveals that the concentrations of weak oxygen bonding and NOV defects with lifetimes of in as-implanted are , agreeing well with those determined using C-V analysis. The NOV and weak oxygen-bonding defects reach their maximum densities and the shortest emission lifetimes of and , respectively, after annealing for . These results support the dissociation of matrix during a Si-implanting process with a reaction of , which is the origin of the strong blue-green CWPL observed in the . The regrowth of matrix after long-term annealing is also confirmed by the significant reduction of NOV and weak oxygen bonding defects. In contrast, the concentration of the center reveals a slowly increasing trend due to the less pronounced precipitation of nanocrystallite Si in during the annealing process.

Oxygen defect and Si nanocrystal dependent white-light and near-infrared electroluminescence of Si-implanted and plasma-enhanced chemical-vapor deposition-grown Si-rich SiO2

The mechanisms for silicon (Si) defect and nanocrystal related white and near-infrared electroluminescences (ELs) of Si-rich SiO2 films synthesized by Si-ion implantation and plasma-enhanced chemical-vapor deposition (PECVD) are investigated. The strong photoluminescence (PL) of Si-ion-implanted SiO2 (SiO2:Si+) at 415–455 nm contributed by weak-oxygen bond and neutral oxygen vacancy defects is observed after 1100 °C annealing for 180 min. The white-light EL of a reverse-biased SiO2:Si+ metal-oxide-semiconductor (MOS) diode with a turn-on voltage of 3.3 V originates from the minority-carrier tunneling and recombination in the defect states of SiO2:Si+, which exhibits maximum EL power of 120 nW at bias of 15 V with a power–current slope of 2.2μW∕A. The precipitation of nanocrystallite silicon (nc-Si) in SiO2:Si+ is less pronounced due to relatively small excess Si density. In contrast, the 4-nm nc-Si contributed to PL and EL at about 760 nm is precipitated in the PECVD-grown Si-rich SiOx film after annealing at 1100 °C for 30 min. The indium-tin-oxide/Si-rich SiOx∕p-Si∕Al metal oxide semiconductor (MOS) diode is highly resistive with turn-on voltage and power-current (P–I) slope of 86 V and 0.7mW∕A, respectively. The decomposed EL peaks at 625 and 768 nm are contributed by the bias-dependent cold-carrier tunneling between the excited states in adjacent nc-Si quantum dots.

Time-resolved photoluminescence and capacitance–voltage analysis of the neutral vacancy defect in silicon implanted SiO2 on silicon substrate

The nanosecond photoluminescence (PL) dynamics of neutral oxygen vacancy (NOV) defects at 410–460nm, and less pronounced nanocrystallite Si precursor (E′δ) defects at 520nm in multi-energy silicon-ion-implanted SiO2 (SiO2:Si+) are investigated. The density of NOV defects in as-implanted SiO2:Si+ of 8×1016cm−3 (or 2.5×1016cm−3 calculated from time-resolved PL) is determined by using capacitance-voltage measurement. After annealing at 1100°C for 3h, the NOV defects are completely activated with a concentration of 4.8×1017cm−3 and a corresponding absorption cross section of 9×10−17cm2. The time-resolved PL lifetime of NOV defects in SiO2:Si+ is significantly shortened from 26to3.6ns and these defects are fully activated after annealing for 3h. Longer annealing time greatly attenuates the blue-green PL intensity and eliminates the NOV defects, whereas the PL intensity and concentration of E′δ defects with lifetime of 20–50ns increases by a factor of 2.

Improved blue-green electroluminescence of metal-oxide-semiconductor diode fabricated on multirecipe Si-implanted and annealed SiO2/Si substrate

The defect-enhanced blue-green photoluminescence (PL) and electroluminescence (EL) of a metal-oxide-semiconductor (MOS) diode made on 500-nm-thick Si-ion-implanted SiO2 (SiO2:Si+) on Si substrate are demonstrated. A multienergy/multidose implantation and 1100 °C annealing process is employed to enhance the 415–455 nm PL contributed by weak oxygen bond and neutral oxygen vacancy defects. The Ag/SiO2:Si+/n-Si/Ag MOS diode exhibits a negative-differential resistance effect with threshold field strength of 300 kV/cm. The threshold pulsed current of deep-blue EL from Ag/SiO2:Si+/n-Si/Ag diode is 280 mA (or 3 V), which turns to white-light emission at saturation current of 680 mA and further shifts to green as the biased current increases up to 3 A. The 3 dB power decay within 3 h is also observed.

Annealing effects on structures and optical properties of silicon nanostructured films prepared by pulsed-laser ablation in inert background gas

We studied the annealing effects on the structures and optical properties of silicon (Si) nanostructured films. The Si nanostructured films were synthesized by pulsed-laser ablation in inert background gas. It was found that the Si nanostructured films partially include an amorphous-like structure under the as-deposited condition. After annealing in nitrogen gas, the crystallinities of the Si nanoparticles recovered, and four visible photoluminescence (PL) bands (1.7, 2.2, 2.7, and 3.1 eV) appeared at room temperature. Furthermore, upon subsequent annealing in oxygen gas, strong quantum confinement effects for both phonons and carriers of the Si nanoparticles appeared, and the relative intensity of the 2.7 eV blue band increased. We fabricated electroluminescent (EL) diodes with active layers of annealed Si nanostructured films, and verified visible EL spectra at room temperature, which exhibited the same trends as the PL spectra. A possible explanation for the increase of relative intensity of the 2.7 eV blue band is the increase of the amount of silicon dioxide, which contains neutral oxygen vacancy defects, in the Si nanostructured films.

High temperature proton implantation induced photosensitivity of Ge-doped SiO2 planar waveguides

The possibility of using keV proton implantation at 800 °C to enhance the photosensitivity of Ge-doped silica has been investigated. Room temperature implantation induced defects indicated by absorption at ultraviolet (UV) (<200 nm) and visible wavelengths (>550 nm) were annealed during implantation at 800 °C to leave stable photosensitive neutral oxygen vacancy (NOV) centers with an absorption peak at ∼240 nm. The stable NOV defects were photochemically bleached after UV exposure, a process which is accompanied by a change in UV absorption. Positron annihilation spectroscopy demonstrated the effectiveness of implantation at 800 °C in annealing the implantation induced damage.

Photoabsorption of the neutral oxygen vacancy in silicate and germanosilicate glasses: First-principles calculations

The photoabsorption of the neutral oxygen vacancy defects in silicate and germanosilicate glasses has been studied by first-principles quantum chemical techniques. The lowest singlet-to-singlet excitations in these defects occur around 7.0 eV, and involve promotion of an electron from a bonding orbital between the adjacent Si (or Ge) atoms to a diffuse Rydberg-type orbital. Such excitations are too high in energy to contribute significantly to the 5 eV absorption band in silicate and germanosilicate glasses.

Oxygen vacancy in alpha -quartz: A possible bi- and metastable defect.

Semiempirical cluster calculations are performed on positive (E' 1 center) and neutral oxygen-vacancy defects in α-quartz. A comparison is made of the various available Hamiltonians and the use of unrestricted wave functions. The results predict that these two defects form a bistable system, exhibiting the same two metastable configurations but differing in which configuration is lower in energy. The neutral charge state favors a configuration in which the silicon atoms adjacent to the vacancy relax only slightly away from their perfect crystal positions. The positive charge state yields a minimum energy configuration with large relaxation of the silicon on the long-bond side of the vacancy